Many players aren't aware of how Berserk Skill actually works. As a result they may put it on a hero who doesn't benefit from the skill the way they expect to.
First, one must remember that in battle, all ATK Speed is rounded up to the closest 200. So, for instance, Grizly Reaper has the same ATK Speed as Ninja, because 750 rounds up to 800. Same goes for Assassin, 900 rounds up to 1000 and results in Assassin having the same attack with the same speed as Druid. Of course, all these numbers don't consider buffs, such as Berserk or Celebrate.
The table below shows how Berserk Talent affects different heroes. Numbers in bold text, in the "No Berserk" row, show actual attack speed of a hero without any buffs. Lower rows show Berserk Talent level 1 through 5. Green areas show actual improvement on the attack speed, i.e., when Berserk Talent starts to actually help. Keep in mind, that these numbers don't take Celebrate into considiration.
Berserk Talent vs Heroes' ATK Speed
Conclusions
1) Grizly Reaper, as heroes with ATK Speed 1000, will benefit only from Berserk 4-5/5.
2) Ninja doesn't benefit from Berserk at all (remember, Celebrate isn't considered in this table).
3) The only heroes that benefit twice from Berserk, are the really slow ones with ATK Speed 1500 and 2000 (that also includes all level and stacks of Celebrate Skill applied).